Unique subgraphs are not easier to find

Given a pattern graph H with l edges, and a host graph G guaranteed to contain at most one occurrence of a subgraph isomorphic to H, we show that the time complexity of the problem of finding such an occurrence (if any) in G as well as that of the decision version of the problem are within a multiplicative factor O(l) of the time complexity for the corresponding problem in the general case, when G may contain several occurrences of H. It follows that for pattern graphs of constant size, the aforementioned uniqueness guarantee cannot yield any asymptotic speed up. We also derive analogous results with the analogous multiplicative factor linear in the number of vertices of H in the induced case when occurrences of induced subgraphs of G isomorphic to H are sought.